This invention relates generally to the field of three-dimensional (3-D) seismic prospecting. More particularly, the invention relates to a method for correcting errors in seismic amplitudes created by acquisition geometry.
In the oil and gas industry, seismic prospecting techniques are commonly used to aid in the search for and evaluation of hydrocarbon deposits located in subterranean formations. In seismic prospecting, seismic energy sources are used to generate a seismic signal which propagates into the earth and is at least partially reflected by subsurface seismic reflectors. Such seismic reflectors typically are interfaces between subterranean formations having different elastic properties. The reflections are caused by differences in elastic properties, specifically wave velocity and rock density, which lead to differences in impedance at the interfaces. The reflections are recorded by seismic detectors at or near the surface of the earth, in an overlying body of water, or at known depths in boreholes. The resulting seismic data may be processed to yield information relating to the geologic structure and properties of the subterranean formations and potential hydrocarbon content.
The goal of all seismic data processing is to extract from the data as much information as possible regarding the subterranean formations. In order for the processed seismic data to fully represent geologic subsurface properties, the true amplitudes resulting from reflection of the input signal by the geologic target must be accurately represented. This requires that the amplitudes of the seismic data must be processed free from non-geologic effects. Non-geologic amplitude effects include mechanisms that cause the measured seismic amplitudes to deviate from the amplitudes caused by the reflection coefficient of the geologic target. These non-geologic amplitude effects can be related to acquisition of the data or to near surface effects. Examples of non-geologic amplitude effects that can be particularly troublesome are source and receiver variations, coherent noises, electrical noise or spikes, and overburden and transmission effects. If uncorrected, these effects can distort the seismic image and obscure the true geologic picture.
A seismic wave source generates a wave that reflects from or xe2x80x9cilluminatesxe2x80x9d a portion of a reflector. As used herein, xe2x80x9cilluminationxe2x80x9d means the imaged amplitude that would be achieved by the given seismic acquisition geometry if the reflection coefficient on the reflector were unity everywhere. The collection of sources that comprises an entire 3-D survey generally illuminates a large region of the reflector. Conventional prestack 3-D migration algorithms can produce precise images of the reflector only if illumination is relatively uniform.
In principle, the best seismic acquisition would result in each common offset (source-receiver distance) data volume having dense uniform sampling in midpoint distribution and fixed source-receiver azimuth (compass direction). If this were the case, flat lying reflectors would be uniformly illuminated (assuming velocity is a function only of depth and not of horizontal position) and, apart from noise issues, amplitudes would normally be trustworthy. With controlled amplitude processing, dipping reflectors would also have trustworthy amplitudes.
In reality, midpoint distribution is not always uniform. For example, in marine acquisition there may be gaps due for example to physical obstructions such as platforms. On land, physical obstacles to seismic acquisition are often plentiful and various. For marine acquisition, source-receiver azimuth generally varies within a survey due to multi-streamer acquisition, and/or cable feathering. In the presence of strong ocean currents or with acquisition using large numbers of parallel streamers (up to 20 streamers will be deployed from boats now under construction), azimuth variations within a survey can be large. Such azimuth variations are known to produce large variations in illumination on dipping reflectors. Azimuth variations are built into the acquisition pattern of typical land surveys.
Such irregularities in midpoint distribution and/or azimuth variation generally lead to non-uniform illumination of reflectors and irregularities in the measured seismic amplitudes. Non-uniform illumination also occurs at the edges of a seismic survey as the illumination decreases to zero across a lateral distance comparable in size to a Fresnel zone, which is on the order of a few hundred meters for typical survey parameters. All such non-uniform illumination is either not compensated for in present methods of controlled amplitude migration, or is inadequately compensated for as described below.
Consequently, prestack 3-D migrated images are often contaminated with non-geologic artifacts called geometrical effects or xe2x80x9cacquisition footprintsxe2x80x9d. Unless properly compensated for in the stacking process, the incorrect change of amplitude and signal/noise resulting from these artifacts can interfere with the ultimate interpretation of seismic images and attribute maps. Removing the effects of the acquisition footprint is thus important for accurate seismic acquisition design, processing, and interpretation.
Typical current technology for correcting for acquisition footprints includes xe2x80x9cflex-binningxe2x80x9d. In this approach, the data are normalized to the number of hits in a xe2x80x9cbinxe2x80x9d (typically a rectangle of width equal to the crossline spacing and of length equal to the inline spacing). In the event a bin is empty of data, a trace is copied into the bin from a neighboring bin provided some criterion is met (e.g. the trace comes from not more than half a bin width away in a crossline direction). A basic difficulty with this approach is its inability to deal with gaps more than a bin width in size. In addition, the trace is thereby no longer in its true location.
Another method, proposed by Canning and Gardner (Geophysics 63, 1177-1183 (1998)), uses Voroni weighting over midpoints to correct seismic amplitudes for irregular sampling in midpoint space. This method is computationally expensive for routine application to 3-D data and does not account for varying azimuths typical of multi-cable marine acquisition.
In the method known as EQDMO, which stands for Equal Dip Moveout (ref: Beasley and Klotz, U.S. Pat. No. 5,206,837 (1992)), the data are normalized after dip moveout. As noted by the inventors (ref: 62nd Annual SEG Meeting Extended Abstracts pp 970-973 (1992)), this method encounters difficulties with wide azimuth acquisition. In addition, EQDMO is unable to handle large data gaps, and because the method lacks a true wave theoretical basis, it is unable to guarantee that a uniformly reflective reflector will be correctly imaged.
Therefore, there is a need for a method that works for large data gaps and large variations in azimuth. The present invention is a simple method that satisfies these needs.
In one embodiment, the present invention is a method for correcting errors in seismic amplitudes created by acquisition geometry, comprising the steps of:
(a) calculating the seismic illumination as a finction of offset, depth and x,y position,
(b) scaling each seismic amplitude at each reflection point and offset by a factor equal to the reciprocal of the illumination calculated for such offset, depth and x,y position;
(c) discarding all amplitudes for offsets where the illumination is less than a pre-determined minimum value that is considered desirable for data quality reasons; and
(d) normalizing the remaining seismic amplitudes to compensate for data discarded for quality reasons.
In some embodiments of the present invention, a corrected stack amplitude is obtained, in which process the stack summation is a weighted average over all offsets of the artifact-compensated (step (b) above) amplitudes, with the weighting function that accomplishes steps (c) and (d) above being either zero or unity, depending on whether the illumination is less than or greater than, respectively, the predetermined minimum value. In other embodiments, to eliminate the discontinuity in the weighting function, an intermediate illumination range is set in which the weighting function changes in value from 0 to 1 as a continuous function of illumination. In still other embodiments, that continuous finction is a cosine tail.
In other embodiments of the present invention, the amplitudes are scaled (by the reciprocal of the illumination) after stacking. In still other embodiments of the present invention, instead of the reciprocal of the illumination, some other function of illumination may be used to scale the amplitudes for nonuniform illumination.